The invention relates to an apparatus and a method for exhaust gas aftertreatment for mobile applications.
It is essential to comply with appropriate statutory emission regulations if motor vehicles with spark-ignition and in particular diesel engines are to be used. In this context, catalytic reduction of NOx using hydrogen is regarded as advantageous. This catalytic removal of nitrogen oxides from the combustion exhaust gases of motor vehicles is carried out using hydrogen at suitable catalytic converters in accordance with the reaction 2NO+2H2→N2+2H2O.
In some of the known methods for removing nitrogen oxides by NOx reduction, the hydrogen required for the reaction is carried along in the vehicle, e.g., in pressurized tanks, liquid hydrogen tanks or metal hydride stores. One drawback of this process is that large, heavy tanks are required to carry the hydrogen, and moreover these tanks only have a very limited capacity, which requires short top-up intervals.
EP 537 968 A1 has disclosed an apparatus for the catalytic reduction of nitrogen oxides in exhaust gases from motor vehicles with hydrogen being supplied. The hydrogen is generated onboard the motor vehicle by partial oxidation or reforming of methanol at a suitable catalytic converter. The catalytic converters are heated by virtue of being arranged in the hot exhaust-gas stream from the engine.
DE 101 20 097 A1 has disclosed an exhaust gas purification system in a vehicle with a reforming reactor for extracting hydrogen from fuel, in which system the hydrogen can be fed to an exhaust gas stream in an exhaust pipe an internal combustion engine upstream of an exhaust gas catalytic converter. The reforming reactor has a feed device for oxygen and/or water and is connected to a secondary branch of the exhaust pipe, it being possible for oxygen and water for reforming to be supplied in the form of an exhaust gas part-stream via the secondary branch.
To enable the arrangement of the respective components for exhaust gas purification in the abovementioned patent documents to be implemented, it is necessary to provide a correspondingly large installation space for the exhaust gas aftertreatment apparatus, which is consequently relatively unwieldy.
Therefore, it is an object of the invention to provide a method and an apparatus for exhaust gas aftertreatment which can be used to optimize the installation space so as to effect a more compact design.
The invention solves this problem by providing an exhaust gas aftertreatment device and an exhaust gas aftertreatment method, both of which are described and claimed hereinafter.
The exhaust gas aftertreatment device according to the invention having a reforming unit for generating hydrogen by steam reforming, partial oxidation of hydrocarbons and/or mixed forms thereof is distinguished by the fact that the reforming unit is arranged directly in the main exhaust gas stream from an internal combustion engine. The steam and residual oxygen which are required for the reforming preferably originate from the exhaust gas. The step of providing the required reducing agents consists in briefly switching the internal combustion engine, which is predominantly operated in lean-burn mode and the exhaust gas from which is undergoing the aftertreatment, to rich-burn mode, allowing reforming by means of the reforming reactor according to the invention using the hydrocarbons which are present in the exhaust gas. Various specific measures for controlling the air/fuel ratio, also referred to as the air ratio λ for short, have already been proposed for this purpose, cf. for example laid-open specifications EP 0 560 991 A1 and DE 196 26 835 A1.
In the reforming unit, an exothermic partial oxidation in the presence of residual oxygen or an endothermic steam reforming operation in the absence of oxygen takes place. The combination of the two processes, which is characterized by an excellent heat balance, is referred to as autothermal reforming. Furthermore, the reforming reactor can also be operated as what is known as an autothermal reforming reactor, or ATR reactor for short.
During the reforming, the hydrocarbons in the exhaust gas are substantially converted into a CO—and H2—containing gas mixture (synthesis gas). The reducing agents hydrogen (H2), carbon monoxide (CO) and/or unburned hydrocarbons (HC) which are present here are subsequently used for the reduction of nitrogen oxides.
The apparatus according to the invention and the method according to the invention for exhaust gas aftertreatment, by virtue of the use of a reforming reactor or a reforming unit in the full flow of exhaust gas allow the synthesis gas yield in rich-burn mode to be optimized, which leads in an extremely advantageous way to an improvement to the NOx and sulfur regeneration of the NOx storage catalytic converters and to a reduction in the HC emissions which occur. In addition, the NH3 yield in rich-burn mode on the NOx storage catalytic converter can be optimized.
Cyclical rich-burn mode can be implemented either engine-internally (e.g., afterinjection of fuel into the combustion chamber of the internal combustion engine or throttling), by a secondary injection into the exhaust gas stream upstream of the reforming reactor and/or by a combination of the two options. NOx which is produced in rich-burn mode is substantially broken down by reduction under the reforming conditions.
In lean-burn mode, the reforming reactor behaves like an oxidation catalytic converter which is standard in the exhaust gas sector and reduces the gaseous emissions (HC, CO, NOx) in the oxygen-rich exhaust gas. To allow a rapid cold start, the reforming unit may be equipped with a heating function, e.g., electrical, by means of a flame glow plug, etc.
In accordance with one aspect of the invention, an exhaust gas recirculation may optionally be provided downstream of the reforming unit. In rich-burn mode (λ<1), therefore, it is possible to supply reformate to the engine combustion. This advantageously leads to a drop in the untreated emission levels and at the same time to a lower fuel consumption.
In a refinement of the invention, the at least one exhaust gas catalytic converter, which is preferably an NOx storage catalytic converter which removes nitrogen oxides from lean exhaust gas by storing them as the exhaust gas flows through it and generates N2 by reducing the stored nitrogen oxides when reducing exhaust gas flows through it, is arranged in the main exhaust gas stream downstream of the reforming unit. In addition, NH3 can be generated by selecting suitable operating parameters. Furthermore, at least one further exhaust gas catalytic converter, which is preferably an SCR catalytic converter which reduces nitrogen oxides contained in the exhaust gas using NH3 that has been generated by means of a nitrogen oxide storage catalytic converter or stores excess NH3 and then makes it available as reducing agent in lean-burn mode, is arranged downstream of the NOx storage catalytic converter.
In a preferred configuration of the invention, the at least one exhaust gas catalytic converter, which is preferably an SCR catalytic converter which reduces nitrogen oxides contained in the exhaust gas using NH3 that has been generated by means of the nitrogen oxide storage catalytic converter, is arranged in the main exhaust gas stream downstream of the reforming unit. Furthermore, at least one further exhaust gas catalytic converter, which is preferably an NOx storage catalytic converter which removes nitrogen oxides from lean exhaust gas by storing them as the exhaust gas flows through it and generates N2 by reducing the stored nitrogen oxides when reducing exhaust gas flows through it, is arranged downstream of the SCR catalytic converter.
The use of nitrogen oxide storage catalytic converters, also known as NOx storage catalytic converters or NOx adsorber catalytic converters or NSC for short, is generally known for the post-engine lowering of the levels of nitrogen oxides in internal combustion engines operated in lean-burn mode. Lean-burn operating phases of the internal combustion engine correspond to adsorption phases of the nitrogen oxide storage catalytic converter, in which it oxidizes nitrogen monoxide (NO) to form nitrogen dioxide (NO2) and then stores the latter in the form of nitrates. During brief, periodic regeneration or desorption phases, the stored nitrates are removed from the nitrogen oxide storage catalytic converter by being converted into nitrogen dioxide and then nitrogen monoxide. The latter is then reduced to nitrogen by suitable reducing agents.
A number of problem points are observed in this alternating adsorption/desorption operating sequence. For example, considerable quantities of the polluting gas ammonia (NH3) may form in the regeneration phase as a result of hydrogen reacting with nitrogen monoxide and/or nitrogen dioxide, depending on the catalytic converter temperature, the exhaust gas composition and the material composition of the nitrogen oxide storage catalytic converter. During the transition from a lean exhaust gas atmosphere to a rich exhaust gas atmosphere, there is a risk of undesirable breakthroughs of nitrogen oxides on account of sudden nitrate decomposition if a suitable quantity of reducing agent is not provided sufficiently quickly. During the transition from a rich exhaust gas atmosphere to a lean exhaust gas atmosphere, the nitrogen oxide storage catalytic converter may be heated as a result of exothermic combustion reactions, with the result that nitrates which have already formed may decompose again and temporarily can no longer be stored, which can cause undesirable nitrogen oxide slippage. With this NOx storage catalytic converter technology, efficient lowering of the levels of nitrogen oxides is restricted to a relatively narrow temperature range approximately between 200° C. and 400° C., since at lower temperatures the oxidation of NO to NO2 is inhibited and at higher temperatures the nitrates which have formed can no longer be stably stored in significant quantities and the thermodynamic equilibrium between NO and NO2 increasingly shifts toward the side of the nitrogen monoxide. According to the invention, the provision of synthesis gas expediently results in improved NOx regeneration at a lower temperature, which in turn has an advantageous effect on the ageing properties and efficiency of the NOx storage catalytic converter.
Typical NOx storage catalytic converters contain alkaline-earth metals and alkali metals, which are known for their nitrogen oxide storage capacity. Under lean conditions, the nitrogen oxides are converted as follows:2NO+O2→2NO2 (Pt catalyst)4NO2+O2+2BaCO3→2Ba (NO3)2+2CO2 
Under rich exhaust gas conditions, nitrogen dioxide is desorbed again from the store and directly reacted with the carbon monoxide present in the exhaust gas to form nitrogen oxide:2Ba(NO3)2+2CO2→4NO2+O2+2BaCO3 2NO2+4CO→2CO2+N2 (Pt, Rh-catalyzed)
The switching times between lean and rich operating modes of the engine depend on the quantity of storage material used, the NOx emissions and the parameters which are typical of all catalyzed reactions, such as gas throughput and temperature.
A further problem point when using sulfur-containing fuels is what is known as the sulfur poisoning of the NOx storage catalytic converter caused by the accumulation of sulfates, which are more stable than the nitrates and do not decompose in the NOx regeneration phases. Therefore, special desulfating phases at an increased exhaust gas temperature and a rich exhaust gas composition are usually carried out from time to time in order to remove the sulfates, cf. for example laid-open specification DE 198 27 195 A1. In this case too, in accordance with the invention, the provision of synthesis gas expediently results in improved sulfur regeneration or removal at the NOx storage catalytic converter, likewise at a lower temperature, which in turn has an advantageous effect on the ageing properties of the NOx storage catalytic converter. During desulfating, the polluting gas hydrogen sulfide (H2S) may form; the emission of this gas should be avoided. For this purpose, for example in patent DE 100 25 044 C1, it is proposed that secondary air be fed into the exhaust section during the desulfating phases in order to oxidize the hydrogen sulfide in a subsequent oxidation catalytic converter.
Functions which decide on the need for and possibility of deliberate generation of NH3 and suitably predetermine the operating parameters, in particular the duration and extent of enrichment during the NSC regeneration, are preferably implemented in a corresponding control unit, which, by way of example, can also be used to control the combustion device, such as an internal combustion engine. The formation of NH3 can typically be boosted by using a lower air ratio and a longer regeneration time, provided that the temperature of the NOx storage catalytic converter is in the range in which the formation of NH3 is possible. Furthermore, during NSC regeneration the operation of the combustion device can be set in such a way, in a manner which is known per se, that high untreated NOx emission therefrom is achieved, and as a result the formation of NH3 at the NOx storage catalytic converter is further boosted.
By using a suitable arrangement of the components, it is possible to adapt the maximum thermal loading which occurs in the individual components to the specific requirements. Moreover, by suitable arrangement it can be ensured that the temperatures of the individual components in driving operation are in a range which is favorable for the particular function. The rich-burn mode which is required for regeneration of the NOx storage catalytic converter can be realized by engine-internal measures or an additional post-engine introduction of reducing agents (e.g., fuel into the exhaust section upstream of the reformer), referred to below as secondary injection.
The post-engine supply of reducing agent upstream of the NOx storage catalytic converter can also be used to set rich conditions for NSC regeneration when the engine is operating with a lean exhaust gas. This preferably takes place when the engine is operating at between λ=1.0 and λ=1.2, since otherwise the quantity of reducing agent which needs to be supplied is too great. This has the advantage that a high untreated emission of NOx usually occurs in the range between λ=1.0 and λ=1.2, whereas these emissions are significantly lower at air ratios λ<1. Consequently, this method can be used to achieve a high NOx emission and therefore extensive formation of NH3 during the NSC regeneration.
To avoid high CO and HC emissions during NSC regenerations with λ<1, if necessary secondary air can be blown in upstream of a subsequent oxidation catalytic converter. The secondary air may be provided, for example, by an electrically driven secondary air pump or a compressor or may be removed downstream of the compressor in the case of supercharged engines.
Another known exhaust gas aftertreatment method is what is known as the selective catalytic reduction method, also referred to as the SCR method for short. In this case, to reduce nitrogen oxides, a reducing agent with a selective action, typically ammonia, is added to the exhaust gas. The ammonia is temporarily stored in a corresponding nitrogen removal catalytic converter, referred to as SCR catalytic converter for short, and used by the latter to catalytically reduce nitrogen oxides (NOx) contained in the exhaust gas to form nitrogen and water. At low temperatures, the efficiency of SCR catalytic converters is highly dependent on the NO/NO2 ratio, with a maximum efficiency at an NO2 content of approx. 50% for temperatures below 200° C. and a significantly reduced efficiency if the NO2 content is lower. At higher temperatures above approx. 400° C., the nitrogen oxide reduction is limited by oxidation of ammonia, and moreover the ammonia storage capacity of the SCR catalytic converter decreases as the temperature rises. The overall result for SCR systems of this type is a useful temperature window for efficient lowering of the levels of nitrogen oxides of approximately 250° C. to approximately 550° C. SCR catalytic converters are subject to thermal ageing and should not be exposed to temperatures of over approx. 700° C. to 750° C. The lean-burn phases can be extended by the NH3 stored in the SCR catalytic converter, which advantageously brings about fuel savings and at the same time improved ageing properties of the NSC catalytic converter. It has been found that the SCR catalytic converter can also be used to avoid H2S emission, which occurs, for example, during desulfating. Tests have shown that an SCR catalytic converter, on account of its specific properties, can oxidize hydrogen sulfide which is produced during desulfating to SO2 even with a rich exhaust gas composition (λ<1). This makes it possible to avoid unpleasant odor pollution.
As a further particular feature, SCR catalytic converters can temporarily store hydrocarbons (HC) which are unburned at low temperatures and, if they contain vanadium pentoxide (V2O5), can also oxidize the hydrocarbons under rich conditions (λ<1). It is as a result usually possible to reduce the breakthrough of reducing agent during the NSC regeneration, and also, on account of its property of storing hydrocarbons at low temperatures, to contribute to lowering the HC emission levels after a cold start. In particular the emissions of possibly carcinogenic hydrocarbons, such as benzene, toluene, ethylbenzene and xylene, which may form during rich conditions at the NOx storage catalytic converter, can be lowered. The HC stored at low temperatures are released again at higher temperatures and can be oxidized at the SCR catalytic converter or a downstream oxidization catalytic converter. However, the high temperatures which are required for the oxidization of unburned hydrocarbons at the SCR catalytic converter lead to a deterioration in the ageing properties. This can be overcome by the invention, since the use of the reformer unit in the full flow, with the reformer unit simultaneously functioning as a reformer or reformate-generating unit, allows the synthesis gas content in rich-burn mode to be increased, while at the same time lowering the levels of HC emissions. As a further benefit, this leads to improved ageing properties in the SCR catalytic converter.
Typical SCR catalytic converters contain V2O5, TiO2 and at least one of the components selected from the group consisting of tungsten oxide, molybdenum oxide, silicon dioxide and zeolites.
In a further advantageous embodiment of the invention for post-engine lowering of the levels of particulates, the reforming unit is designed as a catalytic particulate filter. This catalytically active diesel particulate filter is designed as a wall-flow filter. The reforming unit therefore extremely advantageously serves simultaneously as a reformer and as a particulate filter. In addition to the fact that it can be arranged in the full flow of exhaust gas, obviating the need for an additional filter unit, this also leads to a significantly more compact overall design. Moreover, combining or integrating two of the abovementioned functions in one component allows the space required to be considerably reduced.
This particulate filter retains the particulates which are emitted with a high degree of efficiency. The filter can be regenerated by various measures. Since particulates burn off at elevated temperatures, on the one hand the filter temperatures or the exhaust gas temperatures can be increased (e.g., by afterinjection of fuel into the exhaust section), or on the other hand the particulate ignition temperatures can be lowered by catalytic coating or by adding additives to the fuel, to ranges of <400° C., in which case the reduction of the ignition temperature leads to the required afterinjection time being shortened or to a reduction in the afterinjection quantity. A combination of various regeneration methods is also possible. In general, coated particulate filters are far superior to adding additives to the fuel with regard to emissions during the regeneration phase. When the layer of particulates is burnt off, which requires exhaust gas temperatures of over 550° C., CO2 and steam are formed from the particulates. If the exhaust gas which reaches the particulate filter contains NO2, oxidation of particulates by reaction with NO2 is also already taking place in the temperature range from approximately 250° C. to 400° C. (CRT effect).
The reforming unit which simultaneously functions as a particulate filter contains, as support material for an exhaust gas catalytic converter, a ceramic monolith, for example of cordierite, a ceramic with the empirical formula 2MgO×2Al2O3×5SiO2, silicon carbide (SiC) or other suitable materials. The catalytic coating mainly contains support oxides, further oxidic components, such as for example cerium oxide, and precious metals, which are applied to the ceramic monolith as an aqueous coating, also known as washcoat. The support oxides used may, for example, be Al2O3, SiO2, TiO2, zeolites or mixtures thereof, and elements from the rare earths or Zr, optionally in the form of oxides, may also be present in order to increase the specific surface area. In practice, in particular the precious metals have proven to be effective catalysts, in particular Pt, Rh, Pd, Ir, Ru and Ni.
The heating of the NOx storage catalytic converter for desulfating and of the particulate filter for thermal regeneration can be effected by engine-internal measures, including afterinjection of fuel into the combustion chamber. The regeneration of the NOx storage catalytic converter is carried out by means of the H2 and CO formed in the reformer. In addition to the deliberately higher exhaust gas temperature, incompletely burnt hydrocarbons which remain in the exhaust gas lead to additional exothermicity on a catalytic converter which is optionally arranged close to the engine, thereby further raising the exhaust gas temperature. In addition or as an alternative, it is also possible for reducing agents (e.g., fuel) to be supplied in the exhaust section immediately upstream of the component(s) to be heated and/or upstream of an oxidization catalytic converter which precedes these components. This has the advantage that the heat losses caused by having to heat further upstream components and heat losses caused by cooling in the exhaust pipe are reduced. As a result, the energy consumption and therefore the increased fuel consumption for heating are reduced to a minimum. A further advantage is that in this way further upstream components are not exposed to high exhaust gas temperatures, and consequently their thermal ageing can be restricted to a minimum. Moreover, this prevents further upstream components, e.g., an upstream NOx storage catalytic converter, from leaving the temperature window which is required for a good efficiency as a result of being heated.
In the case of a catalytically coated particulate filter, a further advantage is that the conversion of fuel continues to be possible even, for example, after prolonged overrun phases of the internal combustion engine with a low exhaust gas temperature, on account of the high heat capacity of the particulate filter. By contrast, with a conventional catalytic converter, there is a risk of the temperature under similar conditions dropping below the light-off temperature on account of the low heat capacity, so that catalytic conversion of the hydrocarbons is no longer possible. In general, instead of supplying reducing agent (e.g., fuel) upstream of a catalytic converter, it is also possible to use other heating methods instead of supplying reducing agents downstream of the engine. Examples which may be mentioned include electrical heating of the particulate filter/reformer, as measures which are used as standard in practice.
In a particular feature of the invention, the at least one exhaust gas catalytic converter is arranged in the main exhaust gas stream downstream of the reformer reactor, the exhaust gas catalytic converter having the functions of an NOx storage catalytic converter and an SCR catalytic converter. Combining or integrating the two functionalities in one component again allows the space taken up to be considerably reduced.
In a preferred refinement of the invention, an oxidation catalytic converter is arranged downstream of in each case the last exhaust gas catalytic converter.
In a further configuration of the invention, a three-way catalytic converter is arranged immediately after the reforming unit, as seen in the main direction of flow of the exhaust gas.
In another advantageous refinement of the invention, the at least one exhaust gas catalytic converter, which is preferably a DENOX catalytic converter, is arranged in the main exhaust gas stream downstream of the reforming unit. The DENOX catalytic converter may, for example, contain zeolite, Al2O3 and/or perovskite as support material, and for example Pt, Cu or other suitable metals as catalytically active components.
According to a further advantageous design of the invention, an NOx storage catalytic converter is arranged upstream or downstream of the DENOX catalytic converter.
The method for operating an exhaust gas aftertreatment device of the invention allows reduction of nitrogen oxides in exhaust gases from motor vehicles by reduction at a catalytic converter in which hydrogen is supplied, the hydrogen which is required for the nitrogen oxide reduction being generated onboard the motor vehicle by steam reforming, partial oxidation of hydrocarbons and/or mixed forms thereof. In this case, according to the invention, the reforming is carried out directly in the main exhaust gas stream from an internal combustion engine. The steam and residual oxygen required for the reforming preferably originate from the exhaust gas.
In a preferred configuration of the method, the temperature of the reforming unit is set by means of the air/fuel ratio, with the current oxygen concentration in the exhaust gas being determined with the aid of a wide-band lambda sensor.
In accordance with the invention the reforming unit is operated at an air/fuel ratio in the range from approximately 0.5<λ<1.0.
Furthermore, in accordance with a refinement of the method, a quantity of fuel which is fed to the reforming reactor is set engine-internally, by means of a secondary injection and/or by a combination of the two options.
It will be understood that the features described above and those which are yet to be explained below can be used not only in the combination given in each instance but also in other combinations or as stand-alone features without departing from the scope of the present invention.
Further advantages and configurations of the invention will emerge from the claims and the description. In particular, advantages result from a suitable combination or integration of various catalytic converter components as explained below.
The invention is explained in more detail below with reference to the drawing.